any benefits, does not pose any new problems, and does allow correction of an
anachronism in the current criteria.
Container Fill Gas
1. The stored material condition should not change significantly because of reactions
with the container atmosphere. If material stabilization has to be repeated, there
would be additional handling and unnecessary worker radiation exposure.
2. The container atmosphere must not act to mask leak testing and must support leak
Mass of Contained Materials
1. The mass limit for fissile materials is based on criticality safety limits for plutonium.
Pu that is fully water-reflected and
The fissile mass of an isolated sphere of pure
has a 0.05 margin in keff is calculated to be 4.53 kg (9.98 lb.). The parameter keff
indicates the criticality status of an assembly of fissile and possibly other materials
a value of 1.0 indicates the critical condition (a self-sustaining reaction) and values
less than 1.0 indicate subcriticality, with smaller values signifying greater departures
from criticality. A margin of 0.05 (a keff of 0.95) is commonly used to assure
subcriticality. In other words, the smallest amount of plutonium that could go critical
is somewhat more than 4.53 kg. The 4.4 kg limit specified corresponds to the limit
for some shipping packages and allows a modest additional margin of safety. Note
Pu. This constraint
that the mass limit applies to all fissile species and not just
prevents potential criticality incidents involving stored fissile materials (i.e.,
Np, or higher plutonium isotopes) because the critical masses of these fissile
radioisotopes are greater than that of
In terms of plutonium mass, 5.00 kg (11.02 lb.) of plutonium oxide is equivalent to
4.40 kg (9.70 lb.) of plutonium metal. The oxide weight limit refers to the total mass
of the plutonium-bearing materials present, not just to the plutonium oxide content.
This constraint provides additional assurance of subcriticality by making the
conservative assumption that all the contents are pure plutonium dioxide. Mass limits
may be further limited by facility-specific considerations including administrative
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